Clinical manifestations of neurotoxicity following high level pesticide exposure are well known, but there is growing concern that pesticides may also produce subclinical neurologic deficits, particularly as a delayed consequence of a poisoning episode or in situations of chronic, low-level exposure. Such effects might degrade an individual's quality of life or be a prelude to clinically detectable dysfunction or frank neurologic disease. Few studies have investigated the neurologic effects of chronic low-dose exposure. Farmworkers are likely to be highly exposed to pesticides, compared to other occupational groups, but their health has never been adequately studied, partly because of the logistical problems involved. We studied pesticide exposure and neurologic dysfunction among a group of farmworkers in central Florida in order to address the scientific issue of pesticide neurotoxicity and to investigate health effects of pesticides in this underserved group. The specific aims of the study were (i) to determine whether chronic occupational exposure to pesticides impairs sensory, motor or cognitive function evaluated using a battery of neurobehavioral tests; and (ii) to determine whether genetic susceptibility alters the effects of pesticides. We conducted a cross-sectional study comparing pesticide-exposed farmworkers to unexposed controls. Participants in the study came from a farmworker community in central Florida with a stable population. Most farmworkers in this community worked in one of three types of agriculture: growing and harvesting ornamental ferns, working in nurseries, or picking citrus fruit. To gain access to the community, we collaborated with the Farmworker Association of Florida, a local advocacy group. We defined our target population to be members of a local credit union and their spouses; both exposed farmworkers and unexposed controls were recruited from this population. We screened 80% of selected individuals, and 81% of those eligible participated in the study. We recruited 288 individuals who had ever done farm work for at least one month (farmworkers) and 51 controls who did not meet this criterion. Our study is unique in recruiting a large sample from a defined population with a high response rate. Since pesticides potentially affect many aspects of neurologic function, we used a battery of tests to evaluate sensory, motor, and cognitive function. Some tests were implemented on a computer. Tests used were relatively independent of education and cultural background. We collected information on history of farm work, other employment, demographics, lifestyle, and medical history in a structured interview administered in person by trained personnel. We collected and archived buccal cells to use as a source of DNA. Genetic analyses will focus on genes involved in pesticide metabolism, such as paraoxonase. In analyses of the relationship of work history to neurobehavioral performance, we found that ever having done farm work was associated with poor performance on four tests - digit span, finger tapping, Santa Ana, and postural sway - but had little effect on four others - symbol digit latency, vibrotactile threshold, visual contrast sensitivity, and grip strength. The affected tests evaluate cognitive and psychomotor function. Associations with farm work were similar in magnitude to associations with personal characteristics such as age and gender. Longer duration of farm work was associated with worse performance. Associations with fern work were more consistent than associations with nursery or citrus work. Deficits related to the duration of work experience were seen in former as well as current farmworkers, and decreased performance was related to chronic exposure even in the absence of a history of pesticide poisoning. We conclude that long-term experience of farm work is associated with measurable deficits in cognitive and psychomotor function. Our study extends previous findings by demonstrating farm work-related neurologic deficits in a large, population-based study that compared workers in different types of agriculture. The results of this study will contribute to our scientific understanding of the effects of pesticide exposure on neurologic function and also address the health problems of an underserved minority group.